TASK 47 take four: alert ticker visible in the preview + 3 display methods

The creator confirmed the clip fix ("the video plays now"), then asked where the
scrolling text was — it was invisible, for a structural reason. AlertTickerFrame
existed only as a frame-pump callback blitted into the OUTPUT; PreviewPane.xaml had
no element for it, because the strip is master-width and global, not a Source, so it
cannot ride a per-element Image. Nothing was wrong in the renderer: there was no
consumer in the preview. Same class of defect as the missing IsAlertBox trigger, one
layer up (MyMistakes RULE 5/6).

- tickerPreviewSink on AlertOverlayLayer, published from RefreshAlertPreviews() so it
  is always the UI thread; MainViewModel.AlertTicker writes it into one reused
  WriteableBitmap bound to a new global AlertTickerElement, mirroring SocialBarElement.
- Source.AlertDisplayMethod + panel "Display" selector: TickerScroll / Flash / Solid.
  Flash pulses 0.5s on / 0.5s off for the whole alert; Solid is centred and still.
- The marquee was also unreadable: a fixed 140px/s took ~17s per pass, so a 10s alert
  showed the text once, entering from the right and never crossing. Paced in reads per
  alert instead (TickerReadsPerAlert = 3 inside the alert's own length, speed derived
  from it) — never px/s. Research (websearch: how do OBS/Streamlabs/StreamElements
  alert boxes present announcement timing?) settled the unit: Streamlabs exposes "Alert
  Duration: choose how long your alert stays on your stream" and "Text Delay", never a
  scroll-speed slider (https://support.streamlabs.com/hc/en-us/articles/52499995174299-Setting-up-Your-Streamlabs-Alerts).
  Run is phase-started half a frame in so the first frame isn't blank.
- Persistence: AlertDisplayMethod INTEGER NOT NULL DEFAULT 0 via the idempotent
  table_info migration, appended LAST in the SELECT because the Source reader is
  positional (GetInt32(32..34)) — a mid-list insert would silently shift a neighbour.

Tests: 15 new facts (suite 339/339). RealApp STA host: the pane draws the strip and
collapses at alert end; the layer publishes a real 1920x48 frame for all three methods
and nothing when the ticker is off; three passes counted in 10s by the pill's leading
edge resetting (a seamless marquee never blanks, so an empty frame cannot count a pass);
Solid byte-identical at every moment; Flash on for half of each second; the panel shows
and writes back the choice; the DB round-trips all three alert fields together.

Incidental finding: a bound ItemsSource ComboBox in LeftPanel.xaml broke
LayerReorderPersistenceTests.RealMouseDrag (that test injects PHYSICAL mouse input, so a
load-time re-measure moves the rows out from under the cursor). Rewritten as inline
ComboBoxItems, the shape the chat Font selector already uses in that panel. Recorded as
MyMistakes RULE (8).

Models/Source.cs

@@ -40,6 +40,19 @@ public enum ClipShape
     Round
 }
 
+/// <summary>How an alert box presents its announcement strip (TASK 47). The strip is
+/// the full-width pill naming the event ("Viewer — Super Chat · $12.34").</summary>
+public enum AlertDisplayMethod
+{
+    /// <summary>Marquee: the pill scrolls across the top edge, paced so the text is
+    /// read <see cref="AlertOverlayLayer.TickerReadsPerAlert"/> times per alert.</summary>
+    TickerScroll,
+    /// <summary>Pulse: the static pill blinks on and off for the whole alert.</summary>
+    Flash,
+    /// <summary>Static: the pill sits centred and still for the whole alert.</summary>
+    Solid
+}
+
 /// <summary>
 /// A multi-instance scene object: image/background/text (screen/window later).
 /// The webcam is NOT a Source — it's a singleton resource whose per-scene usage
@@ -218,6 +231,13 @@ public class Source : SceneElement
         set => Set(ref _alertVideoVolume, Math.Clamp(value, 0.0, 1.0));
     }
 
+    private AlertDisplayMethod _alertDisplayMethod = AlertDisplayMethod.TickerScroll;
+    public AlertDisplayMethod AlertDisplayMethod
+    {
+        get => _alertDisplayMethod;
+        set => Set(ref _alertDisplayMethod, value);
+    }
+
     public bool IsAlertBox => Type == SourceType.AlertBox;
 
     // Chat box config (TASK 3.18)